The invention relates to a method of resynchronizing data transfer between two modems connected by a dedicated line following a fault caused by a ringing signal, whereby at least one of the modems is assigned to a central office and the other modem is assigned to a remote subscriber.
To an ever-increasing extent dedicated lines between a central office and remote subscribers are being put to use for data transfer. Via these dedicated lines, consisting of the usual two-core telephone line, voice signals are transferred in addition to the data, meaning that in addition to normal telephone calls data can be handled by these lines. Simultaneous transfer of data and voice signals can be implemented without the one influencing the other since transfer is done in different frequency ranges which are no trouble to be kept apart.
However, problems are caused by the ringing signal transferred via the dedicated line from the central office to the remote subscriber since this ringing signal as compared to the other signals transferred has a very high level and thus results in interference and even interruption of the data transfer. The modem connected to the dedicated line at the central office end and the modem connected to the dedicated line at the subscriber end are no longer synchronized to each other on termination of the ringing signal and thus no longer permitting safe data transfer. To get round this problem it is necessary to not only implement resynchronization in which it is ensured that the subscriber modem is able to xe2x80x9cseexe2x80x9d the characteristic structure of the transfer data, i.e. the start of data blocks and groups thereof but also, by means of time-intensive sensing procedures, to sense the parameters of the transfer line which also affect the data stream being transferred. It is particularly this sensing of the line parameters that takes up a lot of time so that roughly 10 seconds are lost by known resynchronization methods until data transfer can be reassumed after a ringing signal.
The invention is based on the object of minimizing the interruption of data transfer via the dedicated line caused by transfer of a ringing signal.
This object is achieved in accordance with the invention by a method of resynchronizing data transfer between two modems connected via a dedicated line after an interruption caused by a ringing signal in which at least one of the modems is assigned to a central office and the other modem is assigned to a remote subscriber, comprising the following steps:
a) setting a ringing ID in the central office modem as soon as a ringing signal is to be sent to the subscriber modem connected via the dedicated line;
b) transferring a ringing alert to the connected subscriber modem;
c) interrupting transmission of the pilot tone accompanying the data transfer set by the central office modem and interrupting clock frequency synchronization implemented during data transfer in the subscriber modem with the pilot tone as reference;
d) signalling the central office modem and subscriber modem on hold to save the characteristic parameters for the momentary data transfer in the two modems followed by transfer of the ringing signal via the dedicated line to the subscriber modem;
e) resetting the ringing TDin the central office modem;
f) resending the pilot tone via the central office modem and reinstating synchronization of the clock frequency in making use of the pilot tone as reference;
g) reestablishing the data transfer mode existing prior to step d) in making use of the characteristic parameters as saved before;
h) synchronizing the subscriber modem to the data stream coming from the central office modem by searching for and detecting specific code groups occurring regularly in the transferred data stream.
In the method in accordance with the invention the subscriber modem is alerted by the central office as to the transfer of a ringing signal. Since in the method in accordance with the invention characteristic parameters of the momentary data transfer are saved prior to sensing the ringing signal, it is possible to reinstate the mode existing before, after the ringing signal, without it being necessary to implement the time-consuming business of sensing the transfer parameters, especially the line parameters so that the procedures taking up most time in resynchronizing are now no longer needed. All that is still needed is to implement sensing the characteristic code groups in the data stream to resynchronize the subscriber modem to the central office modem.